It is known to manufacture paper by supplying the papermaking stock to the forming section of a paper machine under acid, neutral or alkaline conditions. Alkaline conditions are primarily used with a wood free fiber and at a pH in the range of about 8 whereas in acid papermaking from wood containing stock, the pH is generally well below 7 and normally in the order of 4 to 6. Neutral papermaking is usually carried out at pH of about 6.8 to 7.5.
One of the major advantages of alkaline papermaking is that it permits the use of calcium carbonate which is a relatively inexpensive filler in relatively large quantities in the paper. Calcium carbonate is generally less expensive than wood fiber and has a relatively high brightness which thus facilitates the production of low cost priming papers.
Attempts have been made to utilize calcium carbonate filler with acid papermaking and/or neutral papermaking by means of specific acid tolerant carbonates which are specially prepared, for example, by the addition of phosphorus compounds to the surface of the carbonate to stabilize the carbonate under acid conditions. It is also known when using such acid tolerant carbonates to operate the paper machine at pH generally below 7 (i.e. mild acid conditions) and to use phosphoric acid to maintain the pH of the stock.
Chalk has been used as a filler in paper.